oldglo

Hey guys, I have a 2005 RX330 and I think there is an electrical issue with it. If I don't drive it for 2 or 3 days, it completely kills the battery, to the point that it can't even be charged up. Im already on my third battery. What do you think could be the culprit here? And how to do I go about diagnosing it? Unfortunately, I don't know much about the car and why it started doing this as I just bought it. Looking forward to hearing your response! Thanks in advance!